THE SPINAL CORD. 457 



mechanic regularity and precision, so long as the conditions of the 

 experiment remain the same. 



In warm-blooded animals similar results may be obtained for a 

 short time after division of the cord, especially if artificial respiration 

 is maintained and the circulation of the blood continued. The cord 

 will then retain its irritability for some time. If the conditions of 

 experimentation were favorable, it is highly probable that the human 

 spinal cord would execute similar movements. Thus it was observed 

 by Robin in a man who had been decapitated that reflex muscle con- 

 tractions could be elicited by stimulating the skin after the lapse of an 

 hour after execution. "While the right arm was lying extended by 

 the side, with the hand about 25 centimeters distant from the upper 

 part of the thigh, I scratched with the point of a scalpel the skin of the 

 chest at the areola of the nipple, for a space of 10 or n centimeters 

 in extent, without making any pressure on the subjacent muscles. 

 We immediately saw a rapid and successive contraction of the great 

 pectoral muscle, the biceps, probably the brachialis anticus, and 

 lastly the muscles covering the internal condyle. The result was a 

 movement by which the whole arm was made to approach the trunk, 

 with rotation inward and half- flexion of the forearm upon the arm; 

 a true defensive movement, which brought the hand toward the 

 chest as far as the pit of the stomach. Neither the thumb, which 

 was partially bent toward the palm of the hand, nor the fingers, 

 which were half bent over the thumb, presented any movements. 

 The arm being replaced in its former position, we saw it again 

 execute a similar movement on scratching the skin, in the same manner 

 as before, a little below the clavicle. This experiment succeeded 

 four times, but each time the movement was less extensive; and at 

 last scratching the skin over the chest produced only contractions in 

 the great pectoral muscle which hardly stirred the limb" (Dalton). 



Laws of Reflex Action (Pfluger). 



1. Law of Unilaterality. If a feeble irritation be applied to one or 



more sensory nerves, movement takes place usually on one side 

 only, and that the same side as the irritation. 



2. Law of Symmetry. If the irritation becomes sufficiently intense, 



motor reaction is manifested, in addition, in corresponding 

 muscles of the opposite side of the body. 



3. Law of Intensity. Reflex movements are usually more intense 



on the side of irritation; at times the movements of the opposite 

 side equal them in intensity; but they are usually less pronounced. 



4. Law of Radiation. If the excitation still continues to increase, it 



is propagated upward, and motor reaction takes place through 

 centrifugal nerves coming from segments of the cord higher up. 



5. Law of Generalization. When the irritation becomes very intense, 



it is propagated in the medulla oblongata; motor reaction then 



